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In the fruit fly Drosophila melanogaster, a female heterozygous for three genes (a⁺b⁺c⁺ on one…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

In the fruit fly Drosophila melanogaster, a female heterozygous for three genes ( $a^{+}b^{+}c^{+}$ on one chromosome and $abc$ on the homologue) is test crossed to a male that is homozygous recessive for all three. The gene order on the chromosome is not known. The genes are written below in the order $a, b, c$ . Of 2000 progeny the classes are:

$a^{+}\,b^{+}\,c^{+}$ : 690 progeny
$a\,b\,c$ : 658 progeny
$a^{+}\,b\,c$ : 131 progeny
$a\,b^{+}\,c^{+}$ : 121 progeny
$a^{+}\,b\,c^{+}$ : 190 progeny
$a\,b^{+}\,c$ : 182 progeny
$a^{+}\,b^{+}\,c$ : 15 progeny
$a\,b\,c^{+}$ : 13 progeny

Work out the gene order from the data first.

How many progeny belong to the double crossover classes?
What is the map distance between $a$ and $c$ ?
What is the map distance between $c$ and $b$ ?
What is the coefficient of coincidence?
What percentage of the progeny are recombinant for the outer genes $a$ and $b$ ?

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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